At the present time, on board satellites there are numerous electronic systems for example for fulfilling observation missions or missions dedicated to terrestrial communication networks. They include notably a module for communication with the ground, a module for controlling and guiding the satellite, an optoelectronic module for observation missions and/or a module for fulfilling communication infrastructure functions. To operate all the electronic systems, current satellites have an energy storage system of the battery type. They also have a solar energy generator which enables the electronic systems of the satellite to be powered when it is exposed to sunlight and which also enables the batteries to be recharged. When the satellite is located in a position in which the sun is eclipsed by the Earth, the electronic systems are powered solely by the batteries.
The progress made in the field of fuel cells has made it possible to envisage installing such energy storage systems on board satellites. Regenerative fuel cell systems (RFCSs) provide an effective self-contained solution for power delivery and are particularly suitable for classes of telecommunication satellites requiring high power for nominal operation. In the prior art, spacecraft comprising an energy storage system of the fuel cell type are known. For example, the document entitled “PEM Fuel Cell Status and Remaining Challenges for Manned Space-Flight Applications” describes such systems. This document describes inhabited spacecraft and are less constrained by space and miniaturization problems than what we find in satellite design.
It is also known to use fuel cells in the automobile field. However, the space field, in comparison with the automobile field, imposes particular operating constraints on the electronic systems and notably for the functions of supplying energy to the systems. For example, mention may be made of the severe operating environment of the electronic systems, notably in terms of temperature, the reliability constraints of the electronic systems, which have to have an extremely low failure rate and constantly have to undergo changes in power supply mode, depending on whether the satellite is in a position in which the sun is eclipsed and, of course, the constraint that the satellite be energy self-sufficient, which constraint must be solved in order for it to fulfil missions of long duration in space.
In order for the problems dictated by space field, more particularly in respect of satellites, to be better understood, we will now briefly describe the operating modes of a telecommunication satellite. After a satellite has been launched, it is placed in a geostationary orbit after a transfer phase during which it performs a succession of elliptical trajectories until reaching the operational trajectory. A distinction is therefore made between a first, launch phase, a second, transfer phase and a third, geostationary orbit phase. During the launch phase and before the deployment of the solar generators during the transfer phase, the operational systems of the satellite are powered by the on-board energy storage system, which system may be of the battery type or of the regenerative fuel cell type. In the case of a regenerative fuel cell or a battery, the system must have a sufficient energy storage level before the launch in order to provide the necessary power for the electronic systems during the launch phase and before the solar generators are deployed. During the transfer phase and the orbit phase, the satellite is capable of recovering energy by means of a solar power generation system when the latter is correctly directed towards the sun. The electronic systems are therefore powered by the latter system when the satellite is exposed to the sun and by the regenerative fuel cell or the battery in the eclipse positions. The function of the power generation system is also to recharge the secondary energy source (battery or regenerative fuel cell in electrolyser mode).
The principle of the fuel cell is such that the solar power is used to carry out the electrolysis and form, from a product, the fuel and the oxidizer, the latter two elements being stored in separate tanks; and the electrochemical reaction between these two elements generates energy by means of a fuel cell. In fact, a fuel cell is not an energy storage means but an energy conversion means, and in this particular case a means of converting solar energy to electrochemical energy.
Consequently, fuel cell systems require the use of tanks for storing the fuel, the oxidizer and the product resulting from the fuel cell reaction. For example, in the case of the most commonly used fuel cell solution of the H2/O2 type, the reactants are maintained in a gaseous state. This means that auxiliary tanks have to be installed. Depending on the storage pressure and the satellite power, the tanks may become very bulky. This constraint consequently increases the overall size and the mass of the satellite for a system in which the complex electronics are already highly constrained. The electrical performance of energy storage systems is measured in Wh/kg, and consequently the increase in mass results in a reduction in satellite performance.